If you develop on AWS you get a supported experience for a long LONG time (see simpleDB which I used and still works even though they don't seem to market it). Same thing with old instance types. S3 etc etc.
With openstack at least a year or two ago - who can seriously stay on top of what is going on there. You could develop something 3-4 years ago and getting it going on the latest open stack = total pain. What exactly open stack was also muddy - lots of ifs/buts/this 5 year old code that ran on vendor X openstack doesn't seem to run today on vendor Y.
Didn't spend much time on open stack though - and I know the hype train was / is huge - (AWS killer etc). My own sense - a lot of folks freaked out about AWS and all WANTED openstack to work so they had some big gun to blow up AWS with - but they didn't seem to spend much time talking to actual customers / developers, while AWS certainly did.
They don't market it, and if you create an AWS account after it was deprecated in favor of Dynamo you'd basically never know it existed except for some footnotes in the Dynamo documentation referencing its predecessor.
Which is fine; hats off to AWS for maintaining it for customers for so long.